This invention relates to a write-once type recordable optical recording medium, and particularly, to an optical recording medium that enables the improvement of reliability with respect to long-term storage, the reduction of manufacturing cost, and the recording of data and reproduction of recorded data as desired.
Conventionally, optical recording media such as CDs and DVDs have been used widely as recording media for recording digital data. And, in recent years, next-generation optical recording media that are higher in capacity and higher in data transfer rate are being developed actively.
Such optical recording media can be classified largely into ROM type optical recording media such as CD-ROM and DVD-ROM with which neither additional writing nor rewriting of data can be performed, write-once type optical recording media such as CD-R and DVD-R, with which, though additional writing of data is enabled but the rewriting of data is not enabled, and rewritable optical recording media such as CD-RW and DVD-RW, which enable the rewriting of data.
As is widely known, generally with a ROM type optical recording medium, data are recorded by bits that are formed on the substrate in the manufacturing stage, and with a rewritable optical recording medium, for example, a phase transition material is used as a material of a recording layer and data are generally recorded using a change of optical characteristics based on a change of the phase state.
Also, with a write-once type optical recording medium, a cyanine-based dye, phthalocyanine-based dye, azo dye, or other organic material is used as a material of a recording layer, and data are generally recorded using a change of optical characteristics based on a chemical change, physical change, or both types of change of the organic material.
However, an organic dye may undergo degradation due to illumination of sunlight, etc., and in the case where an organic dye is used as a material of a recording layer, the reliability with respect to long-term storage cannot be improved readily. Thus for improvement of reliability with respect to long-term storage, the use of materials besides organic dyes is required. As an example of an arrangement of a recording layer in which a material besides an organic dye is used to satisfy this requirement, the arrangement described in Japanese Patent Pubilication JP-A-62-204442 is known.
With a write-once type optical recording medium described in this document, a single recording layer is arranged by laminating two inorganic material films, and by illuminating a laser beam, elements that are contained as the main components in the two inorganic material film layers become mixed and eutectic crystallization is made to occur in this mixing process. When the materials of the two laminated inorganic material films are thus mixed to cause eutectic crystallization, the region that undergoes eutectic crystallization becomes different from other regions in optical characteristics, and this phenomenon can be used to perform data recording.
Meanwhile for further increase of the storage capacity, write-once type optical recording media, wherein two or more recording layers are laminated across intermediate layers, have come to be proposed in recent years.
With such an optical recording medium, a laser beam is focused onto any one of the recording layers, and by the laser beam that is set to a recording power Pw being illuminated on the recording layer, data are recorded by the forming of a record mark on a predetermined portion of the recording layer.
Meanwhile, when data that have been recorded in a recording layer are to be reproduced, a laser beam is focused onto the recording layer in which the data have been recorded, and the data are reproduced by the illumination of the laser beam that is set to a reproduction power Pr on the recording layer and the detection of the light amount of the laser beam from the recording layer.
With the write-once type optical recording medium described in JP-A-62-204442, improvement of the reliability with respect to long-term storage is possible. Since two inorganic material films are laminated and the mixing of these two inorganic material films is used to record data, two recording films are required to form a single recording layer. Thus, the manufacturing cost of the optical recording medium becomes high.
Also, with the write-once type optical recording medium described in JP-A-62-204442, since the difference in optical characteristics is not large between a record mark, which is formed by the mixing and eutectic crystallization of two inorganic material films, and other regions, it is difficult to record data in a manner by which reproduction signals of good signal characteristics can be obtained.
These issues also arise with write-once type optical recording media in which two or more recording layers are laminated across intermediate layers. In particular with write-once type optical recording media in which two or more recording layers are laminated, since the cost of manufacturing an entire optical recording medium increases in accordance with the number of recording layers, the development of a recording layer with a simple structure that enables the obtaining of reproduction signals with good signal characteristics was desired.
Also, with a write-once type optical recording media in which two or more recording layers are laminated, when data are to be recorded in or recorded data are to be reproduced from a recording layer at a far side of a light incidence surface, a laser beam must be illuminated onto the recording layer at the far side of the light incidence surface via recording layers closer to the light incidence surface. The recording layers that are closer to the light incidence surface are required to have the characteristics of being high in transmittance with respect to the laser beam and being such that the transmittance with respect to the laser beam does not change greatly before and after the forming of a record mark.
Thus regardless of there being just one recording layer or a plurality of recording layers, the development of an optical recording medium, which enables the improvement of reliability with respect to long-term storage, the reduction of manufacturing cost, and the recording of data and reproduction of recorded data as desired, was demanded strongly.